Pitfall section in readme.md for newcomers

[cklam2]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/until-destroy/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

There are certain pitfalls for newcomers when using this library.

Issue Number: #91 and #109

What is the new behavior?

To avoid ppl searching for potential solutions in the closed issues it would be useful to have a list of tips and tricks for the most important pitfalls. New section Potential pitfalls in readme.md

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[arturovt]

👍

[cklam2]

Thank @arturovt, applied the suggested improvements.

[jaco-terbraak]

Thanks @cklam2